(1) Field of the Invention
This invention relates to a method for coupling two plastic tubelike members to assemble mechanical parts as valve, joint and pipe devices. The two plastic tubelike members on which an annular protrusion or an annular groove are respectively provided are directly coupled by engagement between the annular protrusion and groove, while a part surrounding the annular groove is wholly softened by heating to become elastic, and this produces high coupling accuracy, large coupling force and durability.
(2) Description of the Prior Art
In assembling a diaphragm system valve, bolts F in FIG. 9A or a cylindrical screw G in FIGS. 9B and 9C are used with two plastic constructive parts. In assembling a gate valve, bolts F in FIG. 10A, a cylindrical screw G in FIG. 10B and a coupling ring R in FIG. 10C are used with tubelike members assembling a valve and an operating mechanism. In either cases, assembling cost becomes higher and the coupled part shall be reinforced by thick construction, and a crack may occur on the coupled part on which local stress is applied. That is, rigidity becomes too high in the plastic constructive parts to locally produce a crack because of lost flexibility.